As the HVAC industry meets the rapidly expanding demand for comfort conditioning, it faces challenges created by the high energy and electricity demands of vapor-compression cooling systems. The industry also faces challenges from the need to better control indoor humidity. In many applications, desiccant systems, which can dry air without first cooling the air below its dewpoint temperature and run mostly on thermal energy, will address these challenges.
Desiccant systems fall into two categories: those that use solid desiccant rotors and those that circulate a liquid desiccant over a porous bed of contact media. The liquid desiccant systems can be more efficient than their solid desiccant counterparts, but solid desiccant systems traditionally have had lower maintenance requirements. The higher maintenance requirements for the liquid desiccant systems are often caused by the corrosiveness of the liquid desiccant, which usually is a strong salt solution. Maintenance problems can develop if droplets of the liquid desiccant become entrained in the flow of air that is being dried.
As described in U.S. Pat. No. 6,848,265, the contents of which are incorporated herein by reference in their entirety, one approach to ensuring that droplets of desiccant are not entrained in the flow of air is to apply a hydrophilic wick to the surface of the contact media and then flow the liquid desiccant within this wick. Alternatively, membranes (such as Nafion®, manufactured by DuPont, of Wilmington, Del., USA) and hydrophobic, microporous films (such as those manufactured by Celgard, of Charlotte, N.C., USA) have been used to isolate the liquid desiccant from the flow of air while still allowing the desiccant to absorb water vapor from the air. U.S. Pat. Nos. 5,528,905 and 6,684,649, PCT Application Publication No. WO 2009/094032 A, and Conde-Petit, et al., “Open Absorption System for Cooling and Air Conditioning Using Membrane Contactors,” Final Report, Project No. 10131, Im Auftrag des Bundesamt für Energie, Forschungsprogramm Energie in Gebäuden, July 2008 have all demonstrated air conditioning systems in which a liquid desiccant flows on one side of a membrane or hydrophobic, microporous film and the air that is to be dehumidified flows on the other side.
U.S. Pat. No. 4,194,041 discloses an invention in which a hydrophilic wicking layer is laminated to a hydrophobic membrane. U.S. Pat. No. 6,849,184 uses a membrane that is backed by a hydrophilic layer that wicks the liquid towards the membrane. Although U.S. Pat. No. 6,849,184 was applied to a system for desalinating seawater, a review paper of liquid desiccant technology shows a conceptual drawing of a liquid-desiccant contactor proposed by American Energy Exchange (AEX) in which the liquid desiccant flows in a wick that is separated from the process air by a membrane (Conde-Petit, “Liquid Desiccant-Based Air Conditioning Systems”, 1st European Conference on Polygeneration, Tarragona, Spain, October 2007; FIG. 10).